This application claims the benefit of Japanese Applications No. 11-227005, filed in Japan on Aug. 11, 1999, and No. 2000-173427, filed in Japan on Jun. 9, 2000, both of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a reflective-type soft X-ray microscope in which reflected images of samples are observed using soft X-rays. The present invention also relates to a mask inspection device in which reflective masks used in soft X-ray reduction projection exposure are inspected for defects using the above-mentioned reflective-type soft X-ray microscope. In addition, the present invention relates to a reflective mask manufacturing method using the above-mentioned mask inspection device.
2. Discussion of the Related Art
In recent years, as semiconductor integrated circuit elements have become progressively smaller, reduction projection lithographic techniques using soft X-rays of an even shorter wavelength instead of the conventional ultraviolet light have been developed in order to improve the resolution of optical systems, which has been limited by the diffraction limit of light. Soft X-rays with a wavelength of 10 nm to 15 nm are used as the exposure light source in such techniques.
Since there are no available substances that are transparent in the aforementioned wavelength range, reflective masks have been used in soft X-ray reduction projection lithographic techniques instead of the conventional transmission-type masks. In such reflective masks, to reflect soft X-rays, a reflective film is formed of a multi-layer film on the surface of a substrate having sufficient mechanical strength and surface smoothness. A specified circuit pattern is formed on this reflective film by a layer formed of a substance that absorbs soft X-rays. In soft X-ray reduction projection exposure techniques, an image of the circuit pattern formed on the aforementioned reflective mask is focused on the wafer, which is coated with a photoresist, by means of a projection image-focusing optical system constructed of a plurality of optical elements, such as multi-layer film reflective mirrors, etc. The image is thus transferred to the photoresist on the wafer. Furthermore, since soft X-rays are attenuated by absorption in the atmosphere, the entire light path is maintained at a specified degree of vacuum.
Besides foreign objects adhering to the mask surface and defects in terms of external appearance such as missing portions and excess portions of the circuit pattern, defects in the aforementioned reflective mask include defects in the reflective film itself. As described above, the reflective film is a multi-layer film, and a high overall reflectivity can be obtained by aligning the phases of weak reflected light at the interfaces between the layers laminated in the multi-layer configuration. If local step differences are generated in the reflective film as a result of a bumpy surface profile of the substrate itself, which may exist prior to the formation of the aforementioned reflective film, and/or as a result of foreign objects accidentally incorporated during the process of lamination of the multi-layer film, etc., portions having deviations in periodic structures are formed. Consequently, the phase relationship of the light inside the reflective film is deviated from the design relationship at such portions, resulting in local decrease in reflectivity. Here, a phase difference of 2xcfx80 corresponds to a used wavelength of 10 to 15 nm. Accordingly, local step differences on the order of nanometers cause the reflectivity defects. It has been extremely difficult to observe such extremely small step differences with microscopes using visible light or ultraviolet light, or with electron beam microscopes, etc.
If the actual exposure wavelength used in a soft X-ray reduction projection exposure apparatus is used, all defects, which are transferred to the photoresist on the wafer, can be detected. Thus, an inspection of such a reflective mask for defects should preferably be performed using the same wavelength as the exposure wavelength. Consequently, it is necessary to use a reflective-type soft X-ray microscope in order to inspect a reflective mask for defects.
Transmission-type microscopes and reflective microscopes exist for soft X-ray microscopes. In the reflective-type soft X-ray microscopes, Schwarzschild optical systems are widely used. As shown in FIG. 4, a Schwarzschild optical system is an optical system that generally is formed of two spherical-surface mirrors with concentric spherical surfaces, i.e., a concave mirror 2 and a convex mirror 1. A hole 7 that allows light to pass through is formed in the center of the concave mirror 2. In cases where such an optical system is used in the soft X-ray region as described above, the reflective surfaces are coated with a multi-layer film coating that reflects soft X-rays.
Conventionally, in order to construct a reflective-type microscope using such a Schwarzschild optical system, as shown in FIG. 4, it has been necessary to incline the normal of sample 3 with respect to the optical axis 0 (the central axis of rotation of the optical system) so that mechanical interference of illuminating light beam 5 and optical system mirror tube 19 is prevented. Furthermore, when the sample is inclined relative to the optical axis 0, the visual field that can be observed is limited by the depth of focus of the optical system.
It is known that the diffraction-limit resolution (R) and depth of focus (DOF) of the optical system are determined by the numerical aperture (NA) and wavelength (xcex), and are given by the following equations:
R=0.61 xcex/NA
DOF=xcex/NA2
For example, if a resolution of 70 nm is to be obtained with a light source having a wavelength of 13 nm, NA is 0.11, and the depth of focus (DOF) in this case is approximately 1 xcexcm. Here, if the sample is inclined 45xc2x0, the width of the visual field limited by the depth of focus is a mere 1.4 xcexcm. If the sample is installed perpendicular to the optical axis, a visual field of at least several tens of microns can be ensured, although this also depends on the dimensions and magnification of the optical system.
Imaging is also accomplished to some extent even in the regions outside the visual field limited by the depth of focus; however, since blurring of the image is severe, the diffraction-limit resolution cannot be obtained. For example, an experiment performed with a soft X-ray microscope using a Schwarzschild optical system with such an inclined sample arrangement has been reported in Optics Letters, Vol. 17, (1992) p. 157. In this experiment, an image of only about #200 mesh (pitch: 127 xcexcm) was visible, so that the results did not even remotely approach the resolution (0.1 xcexcm) expected from the wavelength (18.2 nm) and NA (0.1) of the optical system.
Accordingly, the present invention is directed to a reflective-type soft X-ray microscope that substantially obviates the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a reflective-type soft X-ray microscope which makes it possible to observe images with the sample installed perpendicular to the optical axis and with a reflective-type arrangement, using an image-focusing system consisting of a concave mirror and a convex mirror, as typified by a Schwarzschild optical system.
Another object of the present invention is to provide a defect inspection device for reflective masks used in soft X-ray reduction projection exposure, which uses such a soft X-ray microscope.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides, in a first aspect, the reflective-type soft X-ray microscope including an image-focusing optical system formed of a concave mirror and a convex mirror; an illumination optical system that has a light source, a filter and a focusing optical element; and a stage mechanism that carries and moves the observation sample, wherein at least one opening part that is used to transmit the illuminating light beam illuminating the sample is formed in the aforementioned concave mirror, and a reflected image of the sample is focused on a soft X-ray image detector by the above-mentioned image-focusing optical system.
In a second aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the first aspect wherein the surface of the sample is positioned to be substantially perpendicular to the optical axis of the aforementioned image-focusing optical system.
In a third aspect, the present invention provides the reflective-type soft X-ray microscope of the invention having the features of the first or second aspect above, wherein in the concave mirror constituting the image-focusing optical system, a diaphragm is installed in the position on which the illuminating light beam that has passed through the aforementioned opening part is incident after being reflected by the sample.
In a fourth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the third aspect, wherein the diaphragm is formed by forming a light-blocking film on the reflective film formed on the surface of the aforementioned concave mirror, while leaving an opening part that acts as a diaphragm.
In a fifth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the third aspect above, wherein the aforementioned diaphragm is formed by forming a reflective film only in the opening part that acts as said diaphragm.
In a sixth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the third aspect above, wherein the aforementioned diaphragm is disposed between the aforementioned sample and the surface of the aforementioned concave mirror, and is formed by a substrate formed of a light-blocking material or a substrate covered by a light-blocking material, which has an opening part that acts as a diaphragm.
In a seventh aspect, the present invention provides the reflective-type soft X-ray microscope having the features of any one of the first through sixth aspects above, wherein the supporting columns that support the convex mirror constituting the aforementioned image-focusing optical system are disposed so that neither the illuminating light beam that illuminates the sample nor the reflected light beam that is reflected by the sample is blocked by said supporting columns.
In an eighth aspect, the present invention provides the reflective-type soft X-ray microscope including an image-focusing optical system including a concave mirror and a convex mirror; an illumination optical system that has a light source, a filter and a focusing optical element; and a stage mechanism that carries and moves the observation sample, wherein at least one opening part that is used to transmit the illuminating light beam illuminating the sample is formed in the aforementioned concave mirror, and an image formed by scattered light or diffracted light is focused on a soft X-ray image detector by the above-mentioned image-focusing optical system.
In a ninth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the eighth aspect above, wherein the surface of the sample is positioned to be substantially perpendicular to the optical axis of the aforementioned image-focusing optical system.
In a tenth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the eighth or ninth aspect above, wherein in the concave mirror that constitutes the image-focusing optical system, a reflective film is formed on the surface of the aforementioned concave mirror, and a light-blocking film is formed by means of a substance that absorbs the reflected light beam only in the position on which the illuminating light beam that has passed through the aforementioned opening part is incident after being reflected by the sample.
In an eleventh aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the eighth or ninth aspect above, wherein in the concave mirror that constitutes the image-focusing optical system, a reflective film is formed on the mirror except in the position on which the illuminating light beam that has passed through the aforementioned opening part is incident after being reflected by the sample.
In a twelfth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of the eighth or ninth aspect above, wherein in the image-focusing optical system, a substrate including a light-blocking material or a substrate covered by a light-blocking material, which blocks the reflected light beam from the aforementioned sample, is disposed between the aforementioned concave mirror and the aforementioned sample.
In a thirteenth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of any one of the eight through twelfth aspects above, wherein a supporting columns that support the convex mirror constituting the image-focusing optical system are disposed so that the illuminating light beam that illuminates the sample is not blocked.
In a fourteenth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of any one of the first through thirteenth aspects above, wherein the illumination optical system includes a light source of one of a laser plasma light source, a discharge plasma light source, and an X-ray laser light source, a filter that selectively transmits soft X-rays of a specified wavelength, and a focusing optical element that focuses the light beam emitted from the light source.
In a fifteenth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of any one of the first through fourteenth aspects above, wherein the illumination optical system includes a selector that switches the illuminating light to soft X-rays, visible light or ultraviolet light.
In a sixteenth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of any one of the first through fifteenth aspects above, wherein a plurality of the aforementioned illumination optical systems are installed, and a plurality of illuminating light beams that have different wavelengths are respectively caused to be incident on the sample via a plurality of different opening parts formed in the concave mirror constituting the aforementioned image-focusing optical system.
In a seventeenth aspect, the present invention provides the reflective-type soft X-ray microscope having the features of any one of the first through sixteenth aspects above, wherein the image-focusing optical system is a Schwarzschild optical system.
In an eighteenth aspect, the present invention provides a mask inspection device for inspecting a reflective mask to be used in a soft X-ray reduction projection exposure by the reflective-type soft X-ray microscope having the features of any one of the first through seventeenth aspects above, using soft X-rays of a wavelength used in the soft X-ray reduction projection exposure.
In a nineteenth aspect, the present invention provides the mask inspection device having the features of the eighteenth aspect above, wherein the surface of the sample is scanned while the intensity of reflected light, diffracted light or scattered light is detected, and an image is acquired in the area where the detected intensity varies.
In a twentieth aspect, the present invention provides a mask manufacturing method for forming a pattern on a substrate to manufacture a reflective mask, the method including at least a first process in which a reflective film which has a multi-layer film that reflects soft X-rays is formed on the surface of a substrate; a second process in which a light-blocking film that absorbs soft X-rays is formed on the surface of the aforementioned reflective film; a third process in which a resist layer is further formed on the surface of the aforementioned light-blocking film; a fourth process in which a desired reflective or light-blocking pattern is exposed on the resist layer; a fifth process in which the aforementioned pattern is formed by developing the aforementioned resist layer; and a sixth process in which the aforementioned light-blocking film is etched with the aforementioned developed resist layer used as a protective film, wherein in at least one of the first process in which a reflective film is formed on the substrate, the fifth process in which a reflective or light-blocking pattern is formed by developing the resist layer, and the sixth process in which the aforementioned pattern is formed by etching the light-blocking film, work is included in which an inspection is made for phase defects in the multi-layer film that forms the aforementioned reflective film, foreign objects on the surface of said multi-layer film, or defects in the resist layer or in the reflective or light-blocking pattern formed on the reflective mask, using the mask inspection device of the nineteenth aspect above.
In a twenty-first aspect, the present invention provides a mask manufacturing method for forming a pattern on a substrate to manufacture a reflective mask, including at least a first process in which a reflective film which has a multi-layer film that reflects soft X-rays is formed on the surface of a substrate; a second process in which a resist layer is formed on the aforementioned reflective film; a third process in which a reflective or light-blocking pattern is exposed on the aforementioned resist layer; a fourth process in which the aforementioned pattern is formed by developing the aforementioned resist layer; and a fifth process in which a light-blocking film consisting of an inorganic compound, an organic compound or an organic-inorganic compound that absorbs soft X-rays is formed in the areas not covered by the aforementioned developed resist layer, with said resist layer used as a protective layer, wherein in at least one of the first process in which a reflective film is formed on the substrate, the fourth process in which a reflective or anti-reflective pattern is formed by developing the resist layer and the fifth process in which a light-blocking film is formed in the form of the aforementioned pattern, work is included in which an inspection is made for phase defects in the multi-layer film that forms the aforementioned reflective film, foreign matter on the surface of said multi-layer film, or defects in the resist layer or in the reflective or light-blocking pattern formed on the reflective mask, using the mask inspection device of the nineteenth aspect above.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.